The remarkably high strength, modulus, and hardness of iron-based glasses, combined with their low cost, prompted an effort over the last five years to design amorphous steel suitable for structural applications. The alloy development effort yielded glasses with critical rod diameters as large as 12 mm and strengths in excess of 4 GPa. (See, e.g., Lu Z P, et al., Phys Rev Lett 2004: 92; 245503; Ponnambalam V, et al., J Mater Res 2004: 19; 1320; and Gu X J, et al., J Mater Res. 2007: 22; 344, the disclosures of each of which are incorporated herein by reference.) These low-cost ultra-strong materials, however, exhibit fracture toughness values as low as 3 MPa m1/2, which are well below the lowest acceptable toughness limit for a structural material. (See, e.g., Hess P A, et al., J Mater Res. 2005: 20; 783, the disclosure of which is incorporated herein by reference.) The low toughness of these glasses has been linked to their elastic constants, specifically their high shear modulus, which for some compositions was reported to exceed 80 GPa. (See, e.g., Gu X J, et al., Acta Mater 2008: 56; 88, the disclosure of which is incorporated herein by reference.) Recent efforts to toughen these alloys by altering their elemental composition yielded glasses with lower shear moduli (below 70 GPa), which exhibit improved notch toughness (as high as 50 MPa m1/2), but compromised glass forming ability (critical rod diameters of less than 3 mm). (See, e.g., Lewandowski J J, et al., Appl Phys Lett 2008: 92; 091918, the disclosure of which is incorporated herein by reference.)
Accordingly, a need exists for Fe-based alloys with particularly low shear moduli (below 60 GPa) that demonstrate high toughness (notch toughness in excess of 50 MPa m1/2) yet adequate glass forming ability (critical rod diameters as large as 6 mm).